This invention refers to a dual frequency band directional antenna or system which constitutes a combination of two antenna types of predetermined dimensions. One of them is a gain-optimized cavity reflector antenna for the higher frequency band and the other is a loop radiator of approximately the same shape and periphery as the cavity rim edge for the lower frequency band. Although both radiating sources are separately energized, they use the entire cavity structure as their common reflector and together form a combination antenna, whose radiation maxima are directed into the center axis normal to the backwall of the cavity structure over both of their frequency bands.
The optimized cavity reflector antenna with its typical radiation characteristics is discussed in literature, for example, in the paper "A New Class of Medium-Size High-Efficiency Reflector Antennas," by Hermann W. Ehrenspeck, published in IEEE Transactions on Antennas and Propagation, Vol. AP-22, No. 2, March 1974, pp 329-332. The paper teaches that a circular cavity reflector antenna, consisting of a pan-like cavity reflector and a feed, for example, a dipole in the center of the cavity, reaches several distinct gain maxima when its frequency of operation is changed. More specifically, directive gain maxima are obtained, when, the diameter of the reflector is near to 1.35 or 2.35 times .lambda..sub.H, or its periphery near to 4.25 or 7.35 times .lambda..sub.H which is the wavelength of the highest operating frequency; and when the surrounding rim is optimized in its width. A typical example is a circularly shaped cavity reflector antenna as shown in FIG. 1 of the reference publication. The cavity is formed by the planar reflector surface A of diameter D.sub.A and the rim B of width W.sub.B which surrounds the reflector area. The edge of the rim is marked as E. Feed F, shown as a dipole, is located in the normal axis of the cavity at a distance d.sub.F from and parallel to the reflector surface A. The linear dipole feed provides linear polarization. Crossed dipoles or any other radiator that provides the desired polarization response may also be used. Of special interest for the present invention is a cavity reflector antenna with a diameter of near to 1.35.lambda..sub.H. For highest directive gain the antenna's surrounding rim has to be adjusted to approximately 0.4.lambda..sub.H for narrow-band and to approximately 0.3.lambda..sub.H for wide-band optimum gain performance over a frequency bandwidth of approximately 2:1. In the latter case the gain maximum is somewhat lower and the gain-versus-frequency curve is approximately proportional to the reflector area in square-wavelength, i.e., the radiation efficiency of the cavity reflector antenna stays approximately constant over the entire 2:1 frequency band.
The loop antenna, which is used as the second radiating source of the combination antenna according to this invention, also has its typical radiation characteristics described in literature. For diameters smaller than one wavelength the loops are usually considered as magnetic dipoles which have radiation minima in the axis normal to the plane of the loop and maxima in the plane of the loop. Loop antennas with such dimensions are often used for direction finding. They could, however, not be applied to the combination antenna according to this invention, as their radiation maximum does not appear in the required direction normal to the plane of the loop. Fortunately this requirement is met by a loop antenna whose perimeter length is one wavelength .lambda..sub.L of its optimum-gain frequency f.sub.L. If a circular antenna shape is selected, the loop diameter has to be chosen as .lambda..sub.L /.pi.. Loop antennas of this type, either of circular, or square shape can be found in combination with a second loop of a little larger perimeter which serves as a reflector. This arrangement has wide application as transmitting and receiving antennas for radio amateur stations because of its markedly increased gain in the forward direction. It should be mentioned, however, that the one-wavelength resonance of the loop radiator limits its operable frequency bandwidth because the wavelength-related changes in the loop current distribution prevent the occurrence of the radiation maximum in the axis normal to the plane of the loop.
The loop is usually made from wire or tubing or can be a narrow metal strip. The location of the feed points on the loop determines its polarization response. If the loop is energized with out-of-phase currents at preselected feed points, the resulting loop current distribution initiates a horizontally polarized field radiation. The radiation pattern is similar to that of two vertically stacked horizontal dipoles and a marked directive gain increase is noticed in the H plane of the loop radiator. Radiation maxima appear in the normal axis on both sides of the loop, while minima appear in the plane of the loop at angles 90.degree. off its normal axis. If the loop radiator is energized at different preselected feed points, the radiation maxima can still be directed into the normal axis of the loop; but they are now vertically, instead of horizontally, polarized. Switching from the first to the second preselected points permits linear cross polarization. To obtain circular polarization a 90.degree. phase shift has to be introduced at one of the feed points.
According to another method, the loop can be energized by a coaxial cable, whose conductor is connected to a first or second preselected feed point with the cable shield connected to the cavity reflector structure for horizontal or vertical polarization response of the radiation field.
In the combination of the two antenna types the loop is supported by nonconducting spacers at a distance of approximately one-tenth to one-twentieth of the cavity diameter from the edge of the cavity rim. The combined radiating sources form one unit, which, for optimized parametrs radiates or receives two discrete frequency bands with their center frequencies more than one octave apart from each other.